In 2009, in what is becoming a pattern, mating and nest-building by ibises and other wading birds soared, right on schedule, scientists say, two years after the most recent drought. The scientists counted 77,505 nests, up from 18, 418 in 2008.

The jump is a hopeful sign for the birds that once filled the skies of South Florida. They began to thin out in the late 1940s as the United States Army Corps of Engineers began overhauling the Everglades to reduce flooding and to improve farm irrigation. The wading bird population plunged in the 1960s and 1970s as re-engineered water flows disrupted breeding.

Scientists say the come-back began about 10 years ago. At peak times these days, the scientists say, there may be 140,000 or so ibises and other wading birds in the Everglades, down from at least several hundred thousand before the engineers laid into the marshes and mangroves. A plan to restore the Everglades to conditions more favorable for the birds has been developed. But progress has been slow. The scientists say, however, that Everglades’s water managers have been coordinating with bird experts lately to provide conditions conducive to breeding.

The scientists say they are starting from a promising base. “There tends to be a fairly strong pattern between droughts and large wading bird events” or episodes of breeding, said Mark I. Cook, the co-editor of a report from the South Florida Water Management District that included the latest data on mating.

After a drought, extra food becomes available for the birds, scientists say, partly as a result of less competition from bass and other sizable fish and because of a boom in minnows, finger-length crayfish and other creatures that wading birds love. The scientists say big fish die in droughts as the water around them disappears. At the same time, as the soil in the Everglades dries out, the scientists say, it releases nutrients that stimulate the growth of plants and algae-like organisms that minnows and crayfish devour. With heartier meals, the scientists say, the birds have extra energy and zest and breed more. The spindly legs and long curved bills of the ibises turn an even brighter pink than usual.

Mr. Cook of the South Florida Water Management District said data suggests that jumps in nesting occur two years after a drought. In 2002, he said, after a drought in 2000, the nest count soared to more than 60,000 from just under 40,000 the previous year. Smaller peaks were observed in 2004 and 2006, each two years after severe dry spells.

Peter Frederick, an expert on wading birds at the University of Florida, and John Ogden, a biologist now working as the director of bird conservation for Audubon of Florida, studied nesting patterns in the Everglades from 1931 to 1946 and from 1974 to 1998. Nest counts zoomed two years after seven of the eight droughts they observed.

Outrage at the slaughter of the birds inspired the creation of the Audubon Society, a national bird protection organization now known simply as Audubon. In the early 1900s Congress and Florida outlawed the killing of the birds. Scientists say the engineers who changed the water flows in the Everglades had not anticipated that their work would harm the birds.

After a drought, Mr. Cook said, little crayfish are among the first creatures to return. As the drought descends and water recedes in the marshes, the crayfish burrow into the mud. The crayfish can stay alive, Mr. Cook said, as long as they keep damp. The survivors breed, starting a new food cycle. With no big fish around to eat them, Mr. Cook said, “the crayfish population explodes.”

Then, he said, “you get an explosion of white ibises.”

Mr. Cook and his colleagues know more about the eating habits of the ibis than of other birds because when scientists approach ibises, the birds get nervous. One of their defense mechanisms is to throw up. More often than not, the scientists say, the birds regurgitate crayfish. Whether the wood storks are big consumers of crayfish is not clear, Mr. Cook said. They do not throw up.

Mr. Cook said the scientists are much more confident of their count of nests than of the birds themselves. The nests are easy to see from light planes and boats, he said – partly because they are stationary.

Counting birds is another thing. “You go into a colony of roseate spoonbills,” said Jerry Lorenz, the research director of Audubon of Florida from his office in Tavernier in the Florida Keys, “and all you see is pink birds flying away. There is no way you can count them. What you do is walk around and count the nests.”

Mr. Cook said he once worked on a program to attach radio transmitters on ibises so they could be tracked. “After they left the nest,” he said, “we lost them immediately. They flew straight out of the Everglades. We found a few in agricultural areas.” #